Histological sections through the walls of abdominal aortic aneurysms showed scarce and disrupted elastic tissue. The elastin content of the aneurysmal aortic media was only 8.1 +/- 3.2% dry defatted weight (n = 11). The elastin content of grossly normal age and anatomically matched aortic media was 35.0 +/- 3.2% dry weight (n = 4) and the elastin content of severely atherosclerotic, stenosed infrarenal aortic media was 22.0 +/- 7.2% dry weight (n = 6). There was an inverse correlation of elastin content with the elastinolytic activity of aortic media homogenates, r = -0.78. Elastase activity, measured by the hydrolysis of [3H]elastin, was highest in aneurysmal aortic homogenates, 92.1 +/- 43.7 U/mg protein (n = 18), falling to 46.9 +/- 13.3 U/mg protein (n = 13) in severely stenosed atherosclerotic aortic homogenates and 35.5 +/- 11.9 U/mg (n = 6) in grossly normal aortic homogenates. The elastinolytic activity of stenotic aorta contained leukocyte elastase as an important component. In aneurysmal homogenates leukocyte elastase was also found but the increased elastase activity resulted from a protease(s) (Mr 95,000) extracted in 2 M urea, having minimal specificity for alanyl bonds and no immunological cross-reactivity with leukocyte elastase.